The overall objective of this project is to investigate the role of human monocytes and thymus-derived lymphocytes (T-lymphocytes) in the immune surveillance against the tumor development. The role of these two cell populations against the neoplasia is suggested by a known ability of T cells to recognize tumor antigens and subsequently generate its cytotoxic activity through either direct cell to cell contact or the release of effective humoral factor (lymphokine), and a unique ability of transformation of monocytes to aggressive macrophages in response to lymphokine released by T cells. The purpose of this project is to study the interrelationship of T cell markers and the various lymphocyte functions in-vitro on a sequential observation of various childhood malignancy from the early stage through the course of remission or relapse and the effect of chemotherapy or irradiations. Concurrent investigations of the changes of receptors on monocyte and its relattonship to the sensitivity of response to lymphokine and phagocytosis on the same patients will be made. Additional study will e made to define any possible blocking factors in patient's serum which may interfere with lymphocyte and monocyte functions. These findings will be a great aid in establishing a sensitive and dynamic method of monitoring clinical activity of cancer patients as well as for a better understanding of complex human immune surveillance system against the tumor development.